1. Field of the Invention
The present invention pertains to a gas measurement system, and, in particular, to a gas measurement system having an improved optical system, and to a method of using such a system.
2. Description of the Related Art
It is well-known to those skilled in the art that non-dispersive infrared (NDIR) type gas analyzers operate on the principle that the concentration of specific gases, such as carbon dioxide, nitrous oxide, and anesthetic agents, can be determined by (a) directing infrared radiation from an infrared emitter through a sample of a gaseous mixture, (b) filtering this infrared radiation to minimize the energy outside the band absorbed by the specific gases, (c) measuring the radiation impinging upon an infrared radiation detector and which has passed through this sample, and (d) relating a measure of the infrared absorption of the gas to a gas concentration. Gases that may be measured exhibit increased absorption (and reduced transmittance) at specific wavelengths in the infrared spectrum. Moreover, the greater the gas concentration, the greater the infrared absorption and the lower transmittance.
Also well-known to those skilled in the art are dispersive infrared (DIR) gas analyzers, which operate on the principle that the concentration of specific gases can be determined by (a) directing infrared radiation from an infrared emitter through a sample of a gaseous mixture, (b) separating the received radiation into a discrete number of wavelengths in a wavelength band of interest using a prism or grating, (c) measuring the radiation of each wavelength impinging upon a infrared radiation detector and which has passed through this sample, and (d) relating measures of the infrared absorption determined at each of the wavelengths to at least one gas concentration.
NDIR gas analyzers are widely used in medical applications and can be characterized as either disposed in the main path of the patient's respiratory gases, known as mainstream or non-diverting gas analyzer, or located off of the main path, known as sidestream or diverting gas analyzer. DIR gas analyzers are used in some medical applications but presently only sidestream gas analyzers are available.
Regardless of whether the analyzer diverts gas from the main gas flow path or not, the gas to be analyzed must travel through a flow passage in which infrared radiation passes through the gas sample. This portion of the passage, generally known as a sample cell, confines a sample composed of one or more gases to a particular flow path that is traversed by the optical path between the infrared radiation source assembly and the infrared radiation detector assembly. Strictly speaking, the sample cell refers only to the portion of the flow path through which infrared radiation passes. However, the sample cell is also referred to the airway adapter by those in the industry and the terms can be used interchangeably.
The infrared radiation source assembly and the infrared radiation detector assembly are both components of a transducer that may be detachably coupled to the sample cell. Also note that the physical relationship between the optical path and flow path depends upon the specific design of the sample cell. At least one optical aperture in the wall of the sample cell permits infrared radiation to traverse the sample cell. A transmissive window located in each optical aperture confines the gases to the sample cell flow passage and keeps out foreign matter, while minimizing the loss of infrared energy as the infrared beam enters and exits from the sample cell through the transmissive window or windows.
The distance traversed by the infrared radiation in the flow passage of the sample cell is known as the measurement path length. In a sample cell where the optical windows are located on opposite sides of the sample cell, the measurement or optical path length is the distance between the optical apertures of the sample cell. In a sample cell with a single optical window and an infrared reflective mirror, the path length is twice the distance between the optical window and the mirror. At a constant partial pressure or concentration of a gas, as the optical path length increases, a greater quantity of the emitted infrared radiation is absorbed at the wavelength(s) specific to the gas of interest due to the presence of a greater number of molecules of the gas of interest along the optical path. The infrared absorbance can be quantified using Beer's Law, which includes a term for the measurement path length. With a ratiometric measurement approach known in the NDIR art, the ratio of two concurrent measurements, the measured radiation at wavelengths specific to the gas interest, known as the data channel value, and the measured radiation at other wavelengths where little or no absorption occurs, known as the reference channel value, allows the partial pressure of the gas to be determined. Thus, to achieve an acceptably low noise and a fast response time for the gas analyzer, the tradeoffs between the measurement path length and volume of the sample cell must be carefully considered in any sample cell design.
Mainstream gas analyzer designs require that the optical and/or electronic components interface with the subject's airway or respiratory circuit. A mainstream analyzer is typically situated such that the patient's inspired and expired respiratory gases pass through the sample cell onto which a transducer, which includes elements necessary for monitoring respiratory gases, is placed. Typically, the optical path in a mainstream system traverses the flow path, with optical apertures being provided in the wall of the sample cell and aligned along and on opposite sides of the flow passage. This configuration allows the beam of infrared radiation to enter the sample cell, traverse the gases in the flow passage, and, after being attenuated, exit from the sample cell to the infrared detector assembly, which is comprised of optical components to direct the infrared radiation and components to detect the radiation.
In an NDIR type gas analyzer, the detection components include narrow band filters and infrared radiation detectors. In DIR type gas analyzers, the detection components comprise a spectrometer as known in the art. Often such designs have sample cell optical apertures of a constant size that are set by each manufacturer and that are sufficiently large so as not to restrict the beam of infrared radiation. These designs provide an adequate resolution and accuracy for clinical measurements provided that the optical path length is sufficient to enable an adequate signal to be measured even in the presence of a relatively high absorbance, i.e., low transmittance. However, the introduction of different removable sample cells, often with different sized optical apertures for applications requiring lower sample cell volumes (such as pediatric and neonatal monitoring, and sidestream adapters used with mainstream transducers), caused a reduction and offset in the signals measured by the infrared radiation detectors. The observed differences between the different types of sample cells that are used with the same transducer have been termed the “bias” problem, and have hampered progress for improved device performance and seamless interchangeability of sample cells with differently sized optical apertures.
In sidestream analyzer designs, the optical and electronic components are typically positioned at a distance away from the subject's airway or respiratory circuit in communication therewith. U.S. Pat. No. 5,282,473, issued to Braig et al., discloses an exemplary sidestream infrared gas analyzer and sample cell. Sidestream gas analyzers typically communicate with a patient's airway by way of a long sampling tube connected to an adapter, e.g., a T-piece at the endotracheal tube or mask connector positioned along a breathing circuit or a nasal catheter that has been placed in communication with the patient's airway. As the patient breathes, gases are continuously drawn at sample flow rates ranging from 50 to 250 ml/min from the breathing circuit through the sampling tube and into the sample cell located within or near the monitor. The physical relationship between the optical path and flow path for sidestream sample cells varies and depends upon the specific manufacturer. Thus, differences in the sample cell's optical path length will impact the sensitivity and optimal operating range of the system.
In the designs from most manufacturers, the optical path in a sidestream system transverses the flow path, with optical apertures being provided in the wall of the sample cell and aligned along and on opposite sides of the flow passage. In other designs, the flow path is more circuitous, so that the optical aperture can be positioned such that the infrared radiation passes through the gas parallel to the direction of gas flow. In such designs, the optical path length may be much greater than in a conventional transverse design. In both sidestream sample cell designs, the sample cell volume defines the amount of gas that needs to be cleared in order for the system to respond to the changes in the sample. In order for an adequate response time to be obtained, this volume must be minimized. Therefore, it is desirable that the beam of infrared radiation that passes through the flow path be narrow and that energy losses are minimal, so that a sufficient signal can be received at the infrared radiation detectors. A sample cell with a large path length, hence high sensitivity, and a small cross section to keep the volume small, can satisfy these requirements.
A number of commercially available mainstream and sidestream gas analyzers employ an infrared source consisting of an infrared emitter and a polished, parabolic, mirror surface formed in the surface which the emitter faces. U.S. Pat. No. 5,369,277 issued to Knodle et al. discloses an exemplary infrared source, as shown in FIG. 1. Mirror 10 collimates the emitted infrared radiation source rays from infrared radiation source 20 and focuses the collimated rays into a beam that is directed along the optical path of the device. The infrared source rays pass through an optical aperture containing an infrared transmissive window, enter one end of the sample cell, and those that are parallel, or nearly parallel, i.e. having a small angle relative to parallel, to the axis of the optical path through the sample cell, pass out the aperture at the other end of the sample cell into the detector. In conventional sample cells, rays that enter at a larger angle relative to the axis of the optical path may be stopped by the aperture at the detector end of the cell. In practice, the length to cross section ratio is large, so that a relatively small number of rays are able to get though to the detector.
With the development of devices using different sized sample cells with smaller apertures, a need exists to reduce the source energy lost in the sample cell, and the associated loss in signal level from the radiation detector. Methods to reduce the amount of source energy lost through large angle rays are sought, thereby improving the signal/noise ratio and performance of such devices.